Polyolefins, such as polyethylene, polypropylene, polybutene, etc. are highly hydrophobic compounds, making it difficult to apply a variety of coatings to polyolefin substrates. One type of suitable primer for polyolefin structures is based on chlorinated polyolefin, particularly chlorinated polypropylene. While chlorinated polypropylene works well as a primer for a polyolefin having no plasticizer or very little plasticizer, it works substantially less well for highly plasticized polyolefins, i.e., polyolefins containing about 10% or more plasticizer based upon the weight of the polyolefin.
A particular application for highly plasticized polyolefin is formation of airbag covers for automotive vehicles. An airbag will be located so as to deploy during impact and protect an occupant from the effects of collision, for example, a driver's side airbag will typically be located in the hub of the steering wheel. For appearance sake, the airbag has a cover. This cover must be of sufficient weakness to be torn away during airbag deployment.
An airbag deploys very rapidly and with very substantial force. Fragmenting particles, such as fragmenting particles of an airbag cover, could cause serious damage to an occupant, particularly because the airbag deploys in the area of the occupant's face. Accordingly, airbag covers are made of a material which will tear, not fragment, during airbag deployment. Such characteristic of an airbag material cover must hold true over a large ambient temperature range and particularly at cold temperatures down to -40.degree. (C. and F). One suitable material is highly plasticized polypropylene, i.e., polypropylene containing at least about 10% by weight, relative to polypropylene, of plasticizer, such as mineral oil. Typically such highly plasticized polyolefin contains 20 wt % or more plasticizer.
Also for aesthetic purposes, the airbag cover is desirably painted so as to match the decor of the interior of the vehicle. For polypropylene-based substrates, the airbag is first coated with an adhesion promoter or primer and then topcoated with a pigmented paint. This coating system, like the airbag cover itself, must not shatter during airbag deployment (even down to -40.degree.), lest rapidly flying paint flakes injure the vehicle occupants. Rather, the coating system should remain adhered to and tear along with the airbag cover during airbag deployment. Furthermore, the system should have weatherability and durability relative to abrasion to maintain the attractiveness of the airbag cover during the normal life of the vehicle. One type of paint which may be formulated as a topcoat to provide a desired combination of flexibility and weatherability is an isocyanate-cross-linked polyester.
As noted above, chlorinated polyolefin, which is a known adhesion promoter for polypropylene substrates, does not, by itself, function well as an adhesion promoter for highly plasticized polyolefin substrates. It is a primary object of the present invention to provide a primer composition for highly plasticized polyolefins and to utilize such primer on a highly plasticized polyolefin substrate and thereafter apply a topcoat. However, use of the primer of the present invention is not limited to use in highly plasticized polyolefins but is shown to be an excellent primer for hydrophobic polymeric substrates, including non-plasticized polyolefins and elastomers, such as Kraton.RTM. elastomers.